drm/i915/dram: allocate struct dram_info dynamically

drm/i915/dram: allocate struct dram_info dynamically

Allocate struct drm_info dynamically, and convert the struct
drm_i915_private and struct xe_device dram_info member into a const
pointer. Move the struct definition to intel_dram.h, and keep it opaque
to everyone not needing it. This also removes the duplication of the
struct definition.

Reviewed-by: Vinod Govindapillai <vinod.govindapillai@intel.com>
Link: https://lore.kernel.org/r/73625095157346ea0e8614108c9b369208e5df66.1748337870.git.jani.nikula@intel.com
Signed-off-by: Jani Nikula <jani.nikula@intel.com>